The invention relates to a digital transmission system comprising a transmitter and a receiver, for transmitting a wide-band digital signal having sample frequency F.sub.s, for example a digital audio signal, via a transmission medium, and for receiving said signal; and more particularly to such a system which transmits a second digital signal which comprises consecutive frames, and each frame comprising a plurality of information packets, each information packet comprising N bits, N being larger than 1. The receiver comprises a decoder having an input for receiving the second digital signal, and a decoder has an output coupled to an output terminal to supply the wide-band digital signal.
The invention also relates to a transmitter and a receiver for use in the transmission system, to a transmitter in the form of a device for recording the second digital signal in a track on a record carrier, to a record carrier obtained by means of the transmitter, and to a receiver in the form of a device for reading the second digital signal from the track on the record carrier.
A transmission system of the type defined in the opening sentence is known from the article "The Critical Band Coder--Digital Encoding of Speech signals based on the Percentual requirements of the Auditory System" by M.E. Krasner in Proc. IEEE ICASSP 80, Vol. 1, pp 327-331, Apr. 9-11, 1980. This article relates to a transmission system in which the transmitter employs a subband coding system and the receiver employs a corresponding subband decoding system, but the invention is not limited to such a coding system, as will become apparent hereinafter.
In the system known from said publication the speech signal band is divided into a plurality of subbands whose bandwidth approximately corresponds to the bandwidths of the critical bands of the human ear in the respective frequency ranges (cf. FIG. 2 in the article of Krasner). This division has been selected because on the ground of psycho-acoustic experiments it is foreseeable that the quantisation noise in such a subband will be masked to an optimum extent by the signals in this subband if in the quantisation allowance is made for the noise-masking curve of the human ear (this curve gives the threshold value for noise masking in a critical band by a single tone in the centre of the critical band, cf. FIG. 3 in the article by Krasner).
In the case of a high-quality digital music signal, which in conformity with the Compact Disc Standard is represented by 16 bits per signal sample in the case of a sample frequency of 1/T=44.1 kHz, it is found that with a suitably selected bandwidth and a suitably selected quantisation for the respective subbands the use of this known subband-coding system yields quantised output signals of the coder which can be represented by an average number of approximately 2.5 bits per signal sample, the quality of the replica of the music signal not differing perceptibly from that of the original music signal in substantially all passages of substantially all kinds of music signals.
The subbands need not necessarily correspond to the bandwidths of the critical bands of the human ear. Alternatively, the subbands may have other bandwidths, for example they may all have the same bandwidth, provided that allowance is made for this in determining the masking threshold.